1. Field of the Invention
This invention relates to an RF modulator, and more particularly, to an RF modulator built in a VCR (video cassette recorder) that performs TV channel setting through the frequency synthesizer technique.
2. Description of the Related Art
In addition to its main signal processing system, a VCR (video cassette recorder) generally incorporates an RF modulator that RF (radio frequency)-converts the signal from the signal processing system in order to produce a signal suitable for output to the TV (television).
In replaying videotape, the video signal is amplitude-modulated by the amplitude modulator at the frequency generated by an RF carrier oscillator. The audio signal is frequency-modulated by the frequency modulator at the frequency generated by an audio carrier oscillator. The frequency-modulated audio signal is further converted, by a frequency converter, into a frequency generated by the RF carrier oscillator.
The amplitude-modulated video signal and the once-frequency-converted audio signal are added to form an RF signal output, which is in turn transmitted to the TV system.
Channel setting for a playback of videotape is performed by changing the oscillating frequency of the RF carrier oscillator by use of a variable capacitor. The purpose of channel setting is to put the video and audio signals on a vacant TV channel. Each country requires its unique channel setting in the UHF band. To deal with this situation, the variable capacitor is used to cover a wide channel setting range.
To perform channel setting, the user operates the channel setting mode switch. This activates a test pattern generator, whose output signal is connected to the video signal system, with the result that the generator-created test pattern is converted into an RF signal that is supplied to the TV system.
At this point, the TV is set or been set to tune a vacant channel. The variable capacitor is then adjusted so that the test pattern appears on the vacant channel. After the channel setting is complete, the test pattern generator is disconnected to return to the normal mode.
In an RF generator with such an arrangement, the variable capacitor is used to change the oscillating frequency of the RF carrier oscillator for TV channel setting. This raises a problem: use of the variable capacitor results in susceptibility to temperature drift and changes with passage of time in terms of mechanical accuracy, making it impossible to provide stable reception irrespective of the proper channel setting.
Recent VCRs produce signals usable in place of the test pattern for channel setting, such as a signal that makes the entire screen blue or what is called a blue back. This makes it unnecessary to operate the test pattern generator to supply the test pattern.